Big Ideas can be seen as overarching concepts that occur in various mathematical topics and strands within a syllabus. Within our project on Big Ideas in School Mathematics, we developed instruments to measure two Big Ideas: Equivalence and Proportionality. The instruments we developed seek to assess students' ability to see these Big Ideas as common ideas connecting within and across topics, and their ability to apply these Big Ideas in solving problems. In this paper, we discuss the development of items for these instruments.